Measuring devices which make digital measured values available are being used increasingly in automation technology. In the field of numerical controls, which are used for controlling machine tools, for example, this is true especially for position-measuring devices for measuring linear or rotary movements. Position-measuring devices which generate digital (absolute) measured values are referred to as absolute position-measuring devices.
Primarily serial data interfaces are used for transmitting absolute position values, since they make do with only a few data-transmission lines, and nevertheless, have high data-transmission rates. Particularly advantageous are what are referred to as synchronous serial interfaces, which have one unidirectional or bidirectional data line and one clock line. Data packets are transmitted via the data line in synchronism with a clock signal on the clock line. A multitude of standard digital interfaces have gained acceptance in automation technology. For example, popular representatives for synchronous serial interfaces are the EnDat interface of HEIDENHAIN, and a further is known under the name SSI. In addition, asynchronous serial interfaces such as Hiperface are also prevalent.
The SSI interface is described, for example, in European Published Patent Application No. 0 171 579. It is a synchronous serial data interface having one unidirectional data line and one unidirectional clock line. Position values are read out from a position-measuring device in synchronism with a clock signal on the clock line.
On the other hand, European Patent No. 0 660 209 describes the fundamentals of the EnDat interface. It is likewise a synchronous serial interface which, however, besides the unidirectional clock line, has a bidirectional data line. It is thereby possible to transmit data in both directions—from the numerical control to the position-measuring device and from the position-measuring device to the numerical control. The data is transmitted in synchronism with a clock signal on the clock line, as well.
Meanwhile, besides the pure data, (e.g., the position values in the case of position-measuring devices), additional data is also transmitted via digital measuring-device interfaces, a few examples of which are: speed; acceleration; temperature in the measuring device; second, independently generated position value; and status information (warning signals, error signals, etc.).
Particularly in view of the operational safety of automation systems, it is necessary to test the reaction of the system to fault conditions, which are reflected in the data exchanged between the control and the measuring device. An example of this is that the control requests position values from a position-measuring device at defined time intervals via the digital measuring-device interface. The position values are transmitted in the form of data packets to the control. To check whether the instantaneous position value is also an actually newly formed position value, the data packet also contains a second position value, which was formed in the measuring device independently of the first position value and has a defined mathematical relation to it. For example, the two position values differ by an offset, which is known to the control. By comparing the two position values, it may be ascertained in the control whether the offset is actually present or not. In the first case, the data was correctly formed and transmitted; in the second case, an error has occurred either in the measuring device or on the transmission path.
However, whether the control actually recognizes such an error and reacts properly is very difficult to check in practice, e.g., in the case of a actual system. Theoretically, of course, the possibility exists to manipulate the measuring device appropriately, i.e., to substitute manipulated measuring devices for the built-in devices. At the latest, this procedure fails because of high expenditure when several measuring devices or several failure situations are to be tested.